Mechanism Operated Cell (MOC) switches or “MOCs” are electrical auxiliary contacts mounted on the stationary portions of switchgear assemblies containing draw-out circuit breakers. These MOC switches change state with the open/closed state of the circuit breaker. Early implementations of MOC switches were driven directly from the circuit breaker mechanism and thus were firmly retained in either the open or closed position as appropriate.
Previous applications of Mechanism Operated Cell (MOC) operator structures (MOCs) were originally part of old technology interrupting mechanisms which typically had a surfeit of energy and moved relatively slowly between the open and closed positions. As such, the MOCs were firmly attached directly to the circuit breaker operating mechanisms and thus both open and closed positions were fixed and not subject to motion from seismic or other external influences. The opening position was fixed by the open position of the mechanism in this case. New circuit breakers utilizing MOCs or replacement circuit breakers utilizing new technology interrupting methods (e.g., vacuum) often move at a higher velocity and have less excess energy that can be devoted to MOC operation. A variety of methods have been employed to ‘decouple’ the closing of the MOC from the basic circuit breaker operation in order to slow down the MOC motion and reduce the amount of energy consumed by the MOC operation. Often this means that a MOC in the open position is loosely held in position such that during a seismic event, false operation of the MOC switch can occur. Alternatively, friction in the mechanism can prevent the full and complete return of the MOC to the open position. In order to ensure the MOC moves to the fully open position and to prevent false operation of the MOC during a seismic event (e.g., earthquake) is it necessary to ensure sufficient holding force is available in both the closed and open positions. Normally the closed position is secure having been driven to that position, either directly or indirectly, by the circuit breaker operating mechanism. The open position often depends upon the operation of cell mounted MOC return spring; which may not be adequate to hold the MOC in an ‘open’ position during seismic events.
A conventional solution to this problem is to add an additional MOC return spring to the circuit breaker to force the MOC to the open position and hold it in that position. Unfortunately, the use of such a return spring involves adding an additional and continuous load to the MOC operation and thus increases the amount of energy required to be taken from circuit breaker mechanism. In particular, the simplest and most common method of employing such a return spring is as a spring directly opposing the closing of the MOC. In order to accomplish its mission of firmly holding the MOC in the open position when the breaker is open the spring requires a fairly high initial load and the space available for such a spring is typically rather small leading to a high spring rate. Thus any return spring force which opposes the closing is high and continues to increase fairly substantially. Making matters worse, in most applications, the closing of the MOC is also opposed by a cell mounted MOC return spring which has the same characteristics as the newly installed breaker mounted MOC return spring, e.g., a substantial initial load and a spring rate which leads to a rapidly increasing force which also opposes the breaker closing. Furthermore, in most circuit breaker mechanisms, the initial available closing energy is high and is consumed nearly completely by the closing operation itself, leaving relatively little excess energy at the end of the closing stroke. Thus, the available closing forces are initially high but decrease as the closing sequence nears completion. However, the forces opposed to the closing increase as the breaker reaches its final closed position.
Thus, there is a need to provide a force to hold a MOC in its open position firmly when the breaker is fully open, with the force rapidly decreasing or disappearing entirely as the breaker proceeds through its closing operation.